The present invention relates to a method of actuating a fuel cell system and a fuel cell system implementing this method.
In recent years, extensive research and development works have been undertaken for a polymer electrolyte fuel cell (PEFC; hereinafter referred to as a fuel cell FC), in which hydrogen is supplied to the anode and oxygen is supplied to the cathode to generate electricity by an electrochemical reaction therebetween. Fuel cells have been widely adapted to various applications such as a fuel cell vehicle driven by generated power of the fuel cell, and a household power supply, and the extent of the applications will be growing in the future with great expectations.
In order to make such a fuel cell generate electricity in a better condition to thereby obtain a sufficient output power, it is necessary to warm up the fuel cell quickly and to keep the fuel cell at a temperature where the fuel cell operates in a better condition (approximately 70° C. in the case of PEFC). Japanese Laid-open Patent Application No. 2003-297399 (paragraphs [0052]-[0074]; FIG. 1) discloses to start up the fuel cell, if the temperature of the fuel cell is lower than a certain temperature (e.g., 0° C.), in a low-temperature actuation mode in which the flow rate of the coolant is decreased to accelerate the warm-up of the fuel cell, instead of starting up in the normal actuation mode. It should be noted that starting up the fuel cell in this low-temperature actuation mode is so-called low-temperature actuation.
MEA (Membrane Electrode Assembly) which forms the fuel cell has an electrolyte membrane (solid polymer electrolyte membrane). In order to ensure proton mobility (diffusibility), it is necessary to keep the electrolyte membrane of the MEA in a wet condition. For this reason, a reaction gas (hydrogen/oxygen) to be supplied to the fuel cell is humidified when necessary. When the fuel cell generates electricity, water is generated at the cathode and part of the generated water migrates to the anode through the electrolyte membrane.
Since the anode and the cathode are formed by a porous member such as carbon paper, part of the humidifying water or the generated water may be pooled in voids (gaps), which are passages for a reaction gas, leading to a decrease in the output power of the fuel cell.
There has been proposed a technique for purging the fuel cell, for instance, after the fuel cell stops the power generation (generation of electricity) to stand by for the next actuation of the fuel cell. In this technique, a purging gas is supplied to the fuel cell so that the purging gas purges the fuel cell. Herein, the purging gas is a gas for purging out water from the fuel cell having an MEA. For example, air or nitrogen is used as the purging gas. The term “purge” or “purging” indicates to purge out water from the fuel cell using the purging gas.
However, in general, water presented in the anode or the cathode of an MEA, which is formed by a porous member such as carbon paper, is hardly removed by purging. Therefore, in order to purge out (remove) moisture from the MEA, namely, to dry the MEA, etc., it is necessary to prolong the time required for purging after the fuel cell stops the power generation. This leads to an increase in operation time of the compressor, etc. to supply the purging gas over an extended time period, so that the operation noise will be a problem.
Further, in order to operate the compressor over an extended time period after the fuel cell stops the power generation, the size of the external power source has to be enlarged. As a result, the system becomes enlarged in size.
In view of the above, the present invention seeks to provide a method of actuating a fuel cell system, which can decrease the time required for purging after the fuel cell stops the power generation without enlarging the size of the system, and a fuel cell system implementing this method.